1. Field of the Invention
The present invention is related to electrically conductive polyparaphenylene polymers which are prepared electrochemically and which are adaptable for use in chemical sensors and other similar devices.
2. The Background of the Invention
A. Electrically Conducting Polymers
There has been a continuous interest over an extended period of time in the production of synthetic polymers which are electrically conductive. It has long been recognized that if such electrically conducting polymers could be produced (and also retain acceptable polymeric properties such as solution processibility and suitable mechanical properties), the potential uses would be extensive.
For example, it has been suggested that conductive polymers could be a desirable alternative, in a wide variety of situations, to traditional conductive metals such as copper. The replacement of copper wire with a synthetic polymer would have many potential benefits, including a marked decrease in cost. Authors have speculated that possible applications for electrically conductive polymers may include use in batteries, antistatic coatings, solar cells, and in connection with other specific chemical substances in sensors of specific chemicals in liquids or gases.
Electrically conductive polymers would generally have a variety of desirable characteristics when used as a conducting material. Polymers are generally low in density and high in placticity as compared to known conductive materials, such as metals. At the same time, some investigators have speculated that it may be possible to produce a conductive polymer which is melt and solution processible. That is to say, the polymer is capable of being extruded or molded to predetermined shapes, or painted onto various surfaces.
It is clear that if such a material could be produced, it would have a wide variety of important applications in the field of electronics, particularly where it is necessary to produce a conductive substance having a complex shape.
Other potential advantages of conductive polymers can easily be identified. Polymers are generally low in toxicity, whereas conventional conductive materials are often highly toxic. The energy requirements needed to produce polymers are also extremely low when compared to the production of conducting metals such as copper and aluminum. The process of polymer production is relatively nonpolluting, and the polymers produced may well be easier to handle than other conducting substances.
As will be discussed further below, while conventional polymers have the desirable characteristics enumerated above, the few polymers which have been modified to become electrically conductive have become hard to handle, brittle, and insoluble. Thus, their usefulness for the purposes discussed above is presently extremely limited.
It will be appreciated that polymers are generally considered to be insulating rather than conducting. The insulating characteristics of polymers have been attributed to the relatively wide spacing between polymer molecules. As a result, production of a conducting polymer involves a major change in the electrical and chemical characteristics of conventional polymers.
Certain polymers, however, are known to be at least somewhat conductive. For example, (SN).sub.x polymers are known to conduct. Even these polymers, however, are significantly less conductive than are conductive metals such as copper. In addition, they lack the desirable properties of easy preparation, solubility, and safety in handling.
Traditional carbon-backbone polymers, as mentioned above, are generally insulating. However, when these polymers are pyrrolyzed or graphitized (i.e., dehydrogenated at high temperatures), electrically conducting graphite polymers have been produced. This conductivity has been attributed to the closely spaced graphite structure formed by these procedures.
Unfortunately, such graphite polymers have been found to be difficult to prepare in a controlled manner and thus show a wide variation of electrical characteristics. Furthermore, these polymers are not easily processed and handled and have few of the other desirable characteristics identified above.
In order to produce a desirable and usable polymer which is also electrically conductive, the practice has developed of adding various additional substances, referred to generally as dopants, to the polymer. These dopants may convert an otherwise insulating polymer to a relatively conductive material without totally changing the other characteristics of the polymer.
A variety of techniques have been developed to add dopants to a polymer. Some widely used techniques include exposing the polymer to a vapor of the dopant or placing the dopant into a solution of the polymer. Each of these techniques has been found capable of incorporating dopants into conventional polymers.
Dopants used in such processes, as would be expected, generally comprise molecules which are either electron acceptors or electron donors. Polymers may be doped to an "n-type" state by incorporating electron donor dopants into the polymers. The most generally used dopants are usually Lewis bases, such as alkalai metals. Likewise, polymers may be doped to a "p-type" state by incorporating electron acceptor dopants into the polymer. The dopants generally used to dope to a p-type state include Lewis acids, such as iodine, bromine and arsenic pentafluoride.
Despite some success in doping polymers, serious problems have been encountered in the preparation and use of doped polymers. For example, doped polymers are not generally stable in air, nor are they stable at temperatures significantly in excess of room temperature. In addition, the dopants used are generally very toxic, which presents significant problems in handling and preparation.
The dopants also generally cause the mechanical characteristics of the polymer to degrade. For example, doped polymers are often brittle and totally insoluble. As a result, the polymer is difficult to handle in both the liquid and solid states. The advantages and uses of such a polymer are, therefore, limited. This defeats the very purpose of having a conductive polymer--that is, to produce a material having desirable metallic conductivity characteristics, while maintaining desirable polymeric mechanical and chemical properties, such as solution processibility. In addition, until the production of the present invention, it has not been possible to electrochemically grow a doped polymer, so that film size and other characteristics are difficult to control.
One polymer which has shown promise in the doped form is polyphenylene. However, it has been recognized that attempts to produce non-doped conductive polyparaphenylenes have been unsuccessful. See U.S. Pat. No. 4,440,669 to Ivory et al., which is entitled "Electrically Conducting Compositions of Doped Polyphenylenes and Shaped Articles Comprising the Same." Specifically, the Ivory patent recognizes that "Researchers have tried for over twenty years to obtain highly conducting complexes of carbon backbone polymers." Ivory, column 1, lines 49-51. Ivory continues stating that "Previous efforts to obtain poly(p-phenylene) complexes having conductivities as high as 10.sup.-3 ohm.sup.-1 cm.sup.-1 have been unsuccessful." Ivory, column 2, lines 42-44. As a result, it will be appreciated that the production of a conducting carbon backbone polymer without resort to doping has been an elusive goal. In particular, previous attempts to produce conductive polyparaphenylene have failed.
Despite the inability of researchers to produce non-doped conductive polymers, the potential utility of such a product has lead to the production of conductive-doped species in various attempts to produce such a polymer. Examples of doped polyphenylene forms which have been made are: polyparaphenylene sulfide, polymetaphenylene sulfide, monomethyl and dimethyl substituted polyparaphenylene sulfides, polyparaphenylene oxide, polyparaphenoxyphenyl sulfide, and polyparaphenylene disulfide. See U.S. Pat. No. 4,375,427 to Miller et al., which is entitled "Thermoplastic Conductive Polymers." While doped polyphenylenes such as these have been produced having increased conductivities, those polymers have many of the drawbacks mentioned above. These polymers are found to be relatively insoluble, infusible, brittle, and cannot generally be shaped without sintering. Thus, their utility for many applications is extremely limited.
In addition, electrochemical oxidative polymerization of benzene to form polyparaphenylene requires the use of solvents such as hydrogenfluoride (HF) or sulfur dioxide (SO.sub.2). These solvents are unattractive because of safety problems (e.g., corrosiveness and toxicity) and the difficulty in handling these solvents.
Thus, as discussed in the Ivory patent, polyparaphenylene has not been considered capable of convenient electrochemical production, nor has it been considered conducting in its nondoped state as discussed in both the Ivory and Miller patents.
Even in view of the discussion above, certain investigators continue to believe that undoped, conductive polymers may be produced. Specifically, interest has centered around conductive polypyrrole polymers. While conductive polypyrrole has been identified, it possesses many of the undesirable physical and chemical characteristics discussed with respect to doped polymers--the polymer is not solution processible, and it is not stable in air. In addition, the polymer is expensive to produce because of the high cost of the pyrrole monomer.
In view of the foregoing discussion, it would be a very significant advancement in the art to produce polymers which were conducting without altering the polymeric structure and without adding dopants. It would be a further advancement in the art if such polymers could be produced which were soluble in conventional solvents and, thus, solution processible, and which also retained favorable mechanical properties such as plasticity. It would also be an advancement in the art if such polymers could be produced from inexpensive and readily available precursor molecules. Such polymers and the methods of manufacture of those polymers are disclosed and claimed below.
B. Chemically Selective Sensors
One area of particular interest is the area of chemical sensors. Chemical sensors are widely used in a broad spectrum of applications. For example, it may be critical in the chemical and petroleum processing field to sense various species and materials in a particular environment. Small quantities of impurities may destroy the effectiveness of a process. Likewise, the addition of small quantities of a particular substance may be required to assure the effectiveness of a process.
Other applications of sensors are numerous. Sensors may be used to detect air and water pollutants. Certain types of sensors may be employed to detect emergency situations such as fires or leaks of toxic materials. Other uses of sensors include biological system monitoring, process line quality control, military use, and chemical analysis.
It is clear that efficient sensors which are capable of sensing small quantities of specific substances are critically needed. However, it has been difficult to produce sensors capable of sensing small quantities of materials in a solution environment. In particular, it has been difficult to produce electrochemically operated sensors for detecting small quantities of materials. This is the case because it has been difficult to generate a sufficient change in current caused by the presence of a small quantity of a substance and because of the lack of specificity in electrochemical experiments.
As a result, it would be a significant advancement in the art to produce a sensor capable of effectively sensing extremely small quantities of a particular material in a gaseous or liquid environment. In particular, it would be an advancement in the art to provide such a sensor which was electrically activated. It would be a further advancement in the art to incorporate a sensing species into a conductive polymer to form such a sensor, such that sensing molecules in the sensor could be caused to sense specific species in a liquid or gaseous environment by passing a current through the conductive polymer.